Method and means for reducing disturbances in wireless reception



K. H. MEIER 2,220,260

METHOD AND MEANS FOR REDUCING DISTURBANCES IN WIRELESS RECEPTION Nov. 5,1940.

Filed July 20, 1938 2 Sheets-Sheet 1 I lm enlfor: Karl Hannah Mekv Nov.5, 1940. K. H. MEIER 2,220,260

METHOD AND MEANS FCR REDUCING DISTUHBANCES IN WIRELESS RECEPTION Filedmy 20. 1939 2 Sheets-Sheet 2 Inventor:

Patented Nov. 5, 1940 UNITED STATES METHOD AND MEAN 8 FOR REDUCING DIS-TURBANCES IN WIRELESS RECEPTION Karl Heinrich Meier, Zurich, SwitzerlandApplication July 20, 1938, Serial No. 220,412 In Switzerland July 24,1937 4 Claims.

High frequency disturbances which become superimposed. upon atransmission are regarded as particularly troublesome and unpleasant ifthe noise intensity resulting therefrom exceeds 5 considerably themomentary sound intensity of the transmission. With local storms at thereception point, the electrical disturbance phenomena accompanying thestorm may assume such high values that the resulting noise intensityproduces a sensation of pain in the listener. In general conditions areconsiderably better than this since such high disturbance voltages areas a rule only present under abnormal conditions. Nevertheless, evenunder more favourable reception conditions, there may still arise a fewdisturbance voltage peaks per second, which substantially exceed thetransmission in their intensity.

The present invention relates to a method and an arrangement forcarrying this method into effect, by which all suddenly occurringdisturbance voltage peaks which exceedin amplitude the transmitteramplitudes are cut oil or limited in amplitude to such an extent thatthe amplitudes of these disturbance voltages can no longer substantiallyexceed the instantaneous amplitude value of the transmission.

The method of suppressing high voltage peaks is carried out, accordingto the invention, by arranging that the transmission is maintained, bymeans of a control device constructed as a compressor, at a constantsound intensity, that is to say is controlled to a constant meanamplitude value, that is to say is smoothed, that the low frequencycomponents of the transmission, controlled to a constant mean amplitudevalue, are impressed upon the grid circuit of an amplifier tube,connected as amplitude filter, in such a way that the whole straightpart of the grid characteristic is swept over, the amplitude filterbeing impassable on account of cessation or saturation of the anodecurrent by voltages which are greater than the range of grid swing, andthat the low frequency components of the transmission which are allowedto pass through the amplitude filter are influenced by a second controldevice constituted by an expander in such a manner that the smoothing ofthe variations in sound intensity effected by the compressor stage iscompensated for in such a manner that the original variations in soundintensity are reestablished in the transmission.

The high frequency influences upon wireless receiving apparatus, ingeneral known as disturbances of reception, are composed of indi-'vidual, strong, disturbance voltage peaks which become superimposed uponthe signals tobe re ceived.

These disturbance voltage peaks which in general arise from atmosphericdisturbances, arise from the sudden collapse of electric fields in theatmosphere (fork and sheet lightning and the like) Whilst disturbancevoltage peaks, arising from the collapse of electric fields last onlyfor a relatively short time (of the order of 10- seconds), theVariations of intensity arising from differences in sound intensity canbe regarded as very slow. As example it may be mentioned that in a pieceof music which is very rich in contrast, often many tenths of a secondelapse before a piano part of the piece swells to fortissimo. As a ruleit can therefore be assumed that variations in sound intensity whichtake place in less than second will arise extremely seldom.

On the other hand the control process should in any case not take placetoo quickly, if a cutting off of the lower frequencies is to be avoided.The time constant of this control device is therefore determined by thelowest frequency to be transmitted.

It is now clear that, relatively to the time constants of the variationsin sound intensity, effects which can be regarded as exceptionally rapidwill not be afiected by a control arrangement which only compensates forthese variations in sound intensity. Their amplitudes will not,therefore, be reduced to the mean amplitude of the transmission but willextend beyond this. They will therefore be cut off since the gridcontrol range of the amplitude filter is only capable of passing theintensity threshold value of the transmission. There is no noticeabledeterioration of the transmission by these individual disturbance peakswhich are reduced to the mean value of the transmission, since, on theone hand, the time during which these disturbance peaks act is veryshort and, on the other hand, even with very strong disturbances only afew of such disturbance peaks arise per second. This cutting off of thedisturb-ance peaks has therefore for consequence only a distortion ofthe low frequency amplitudes. The effect of this is as if the harmonicdistortion of the low frequency stage were to increase from about 1% to5% and this only during the very short spaces of time of the average ofa millisecond per disturbance peak.

Even disturbances which have an apparently coherent disturbancespectrum, as for example electric railways, show on a high frequency 0s5 cillograph individual disturbance peaks of varying amplitude andfrequency, but once more only a few disturbance peaks occur per second.This is quite clear if it is remembered that with this kind ofdisturbance, disturbances can only arise if an interruption occursbetween the current pick-up shoe and the conductor rail. That theseinterruptions occur only a few times per second, is quitecomprehensible. In any case, considerably more interruptions may occurif the conductor rail has a rough surface. (Extended oscillographicinvestigations of the disturbance spectra of the wireless interferencearising from the Swiss Federal Railway have indicated a maximum of 50-60disturbance peaks per second. This maximum was only present with a veryrippled conductor rail whilst with the conductor rail in good conditionan average maximum of 20 disturbance peaks per second occurred.) v

Figures 1 to 3 show how the method according to the present inventionmay be carried out.

Figure 1 illustrates graphically the individual phases of the methodaccording to the invention;

Figure 2 is a graph showing the operation and eff ectiveness ofrectification;

Figure 3 illustrates graphically the limits of the amplitudes;

Figure 4 illustrates the electric circuit connection of one form ofapparatus according to the present invention; and

Figure 5 shows a modification of the circuit of the receiving station.

. In what follows the mode of operation of one form of the process byway of example will be more fully explained with reference to Figs. 1-3,the first control means I being shown as a compressor and the secondcontrol means 3 as expander. These control means I, 3, which are knownin themselves, consist for example of electron discharge tubes havingexponential characteristics, whereby the amplification of these tubescan be varied within wide limits corresponding to the grid bias applied.

- From the incoming transmitter wave 4 there is derived, in a branchcircuit which is shown, a control voltage, and this is smoothed byresistances and condensers to such an extent that the potentialvariations of this control voltage represent the variations in the soundintensity of the transmission. This control voltage which is to beregarded as a dynamic voltage, is led to the grid circuit of thecompressor I, whereby variations in the sound intensity of thistransmission 4 are compensated, in such a way that with soft passages inthe transmission a high amplification takes place whilst, conversely,the amplification decreases with loud passages, in such a manner thatall acoustic frequencies of the transmission are controlled to the samemean amplitude value, as for example is shown in Fig. 1 with thefrequencies 5. The high frequency 5, controlled to constant meanamplitude value by the compressor I, after subsequent rectification by arectifier which is not shown but which advantageously consists of a hotcathode system, and which operates according to the characteristic ofFig. 2, is ledto an amplitude filter 2, consisting of an electrondischarge amplifier tube. The mode of operation of this amplitude filteris as follows:

The positive terminal of the high frequency rectifier referred to isconnected to the common potential point, or the cathode, of thereceiving system so that there remains only one half 6 of the highfrequency wave whose frequencies swing in the negative potential rangerelatively to the cathode. Onthe grid side of the tube serving asamplitude filter 2, which has a small or no grid bias, is impressed therectified high frequency 6, according to the characteristic diagram ofFig. 3, in which the characteristic 1 represents the function I8. (Eg)Whilst the dotted line 8 corresponds to the grid bias 0 of the tube andthe dotted line 9 is located at the bottom bend of the tubecharacteristic. The mode of operation of the amplitude filter 2 is alsoto be derived from this diagram. The rectified high frequency I0 is soadjusted in amplitude that its low frequency components swing oversubstantially the whole range of the characteristic of the amplitudefilter. If there are superimposed upon this rectified high frequency Ita few high disturbance voltage peaks I I and II, whose amplitudes arenot reduced by the control process to the mean amplitude value of thetransmission, since their duration is at a maximum a few millisecondsper impulse, these practically cannot be further amplified because therange of grid control of this amplitude filter is insufiicient for sucha high voltage, as follows from the diagram of Fig. 3, where thefrequencies It allowed to pass by the amplitude filter 2 are shown.

If individual voltage impulses of large amplitude reach the filter 2,they will swing so far into the negative or positive range of the tubecharacteristic I that the anode current becomes zero or saturated andhence the filter 2 is impassable by such high amplitudes.

The low frequency allowed to pass by the amplitude filter 2 is led to asecond control stage 3, the so-called expander stage which receives thesame grid control Voltage as the compressor stage I, but with thedifference that when the stage controls upward, the stage 3 controlsdownward and vice versa, so that by this expander stage the frequencies4' are so controlled that the original variations of sound intensity ofthe transmission 4 are re-established.

The arrangement for carrying out the method has, according to theinvention, the following members: a control means by which the soundintensity variations of the transmission are so influenced that thetransmission then only consists of different frequencies of the samemean amplitude values; an amplitude filter to which are led thesefrequencies which remain constant in their mean amplitude values, and bywhich individual voltage impulses, which arise from a disturbance andwhose amplitudes are greater than the amplitudes of the receptionfrequencies, are reduced to the amplitude value of the receptionfrequencies; and a second control means to which are led the frequencieswhich are allowed to pass by the amplitude filter, whereby the originalvariations in sound intensity are re-established in the transmission.

Fig. 4 shows a circuit diagram of an embodi ment of such an arrangementby way of example.

Fig. 5 shows a modification of a part thereof.

The high frequency. received from an antenna is, in known manner, afterpassing selecting and amplifying means, amplified by the amplifier tubeI5. This amplifier tube, which is constructed as an exponential tubedelivers its amplified high frequency to a rectifying stage IB, whichdelivers a control voltage for the control of amplification to thisfirst stage. This control voltage, whose time constant is suitablydetermined by the condensers I1 and 20 and also the resistances I8 andI9, is led to the grid circuit of this tube I5 whereby a substantialcompensation of the incident 'compensation.)

transmitter field strength is obtained. (Fading In parallel to therectifier tube I6 is connected a second rectifier tube 24 which alsoreceives the high frequency coming from the anode of the tube l5. Thisrectified high frequency is smoothed by the resistances 25, 26- and alsothe condensers 21, 28, 29, 30 to such an extent that the potentialvariations of this control voltage follow the intensity changes arisingfrom the variations in sound intensity. This control voltage is ledthrough the grid resistances 3|, 32 .to the compressor stage 33 and tothe expander stage 34. The high frequency delivered by the firstamplifier stage I is impressed upon the tuned circuit 35 and after thistuned circuit upon the grid circuit of the compressor tube 33.

The frequencies amplified and controlled to a constant mean amplitudevalue by the compressor means 33 are led through the condenser 37 to theanode of the hot cathode rectifier 38. In parallel to the anode-cathodepath of the rectifier 38 are arranged the load resistance 40 and thecondenser 40. The grid of the amplitude filter 39 is also conductivelyconnected with the anode side of the rectifier 38. This amplitude filteradvantageously consists of a screen grid valve which receives ascreening grid voltage of about 30 volts whilst the anode voltageamounts only to 8 volts. Under these voltage conditions there isobtained a characteristic whose curvature is so small that it canpractically be regarded as rectilinear. This amplitude filter may besuitably biased by means of the cathode resistance 4| with the by-passcondenser 4|. A high frequency choke of 95 milli-Henry has been foundsatisfactory for the anode impedance of this amplitude filter. This canhowever be replaced by a resistance of 4000-6000 ohms without thecharacteristic being thereby substantially altered. The frequenciesallowed to pass by the amplitude filter 39 are impressed upon the gridcircuit of the expander 34. This expander, which may also consist of anelectron discharge tube having an exponential characteristic, has itscathode biased by a resistance 52 relatively to the grid circuit to suchan extent that, when the positive control voltage increases, the gridbias decreases, so that a reversed amplifier characteristic relativelyto the compressor is obtained.

Instead of connecting the high frequency rectifier 38 according to thecircuit diagram of Fig. 4, it is advantageous to connect this rectifier38 according to Fig. 5. The high frequency from the compressor 33reaches the cathode 44 of the rectifier 38 through the condenser 43.This cathode 44 is connected with the common potential point 46 througha high frequency choke 45. So that the high frequency voltage to berectified cannot flow through the capacitive shunt between cathode 44and heaters 41, 4'! of the indirectly heated hot cathode rectifier, highfrequency chokes 48, 49 are inserted in the heater leads. The anode 56of the rectifier 38 is connected conductively with the control grid ofthe amplitude filter 39 and through a suitably chosen resistance 5| tothe common potential point 4%. The mode of operation of this arrangementfollows clearly from the drawings, so that a description thereof issuperfluous.

In the example herein described, the compression is effected in the highfrequency part of the apparatus. It is of course readily possible toarrange the compression stage after demodulation has taken place. Inthis case the amplitude filter should be biased negatively with negativegrid bias to such an extent that the low frequency of the transmissiononce more swings overthe whole straight part of the grid characteristicof this amplitude filter. It is also possible, moreover, to constructthe disturbance removing device in accordance with the drawings, whilstthe frequencies leaving the expander device are then led to a modulatortube which produces superposition of the incoming frequenciesupon thefrequencies of a local generator so that there is obtained anintermediate frequency which can be further amplified as such. It isalso possible to derive the dynamic voltage for amplification controlafter the expander, instead of from a sep-. arate shunt branch as in theexample of Fig. 4.

I claim:

1. Method of suppressing high voltage peaks at a radio receiving stationwhich, due to disturbances, become superimposed upon the transmittedsignal duringwireless transmission, comprising the steps of adjusting,at the receiving station, the signal level of the received signal withsuch a time constant that amplitude fluctuations in the signal caused bythe modulation are reduced, reducing the disturbing amplitudes toapproximately the highest signal amplitude, and thereafter enlarging thesignal for contrast, whereby the amplitude fluctuations of the signalwhich were evened out by said signal level control, are compensated for,so that the original amplitude fluctuations are restored.

2. In a wireless receiver, means for controlling the signal level, saidmeans comprising a compressor having such a time constant as to reduce,at the receiver, the amplitude fluctuations in the signal caused bymodulation, an amplitude filter for reducing the disturbing amplitudesto approximately the highest signal amplitude, a control element forenlarging the signal contrast, whereby the amplitude fluctuations of thesignal, which were evened out by the said signal level control, arecompensated for, so that the original amplitude fluctuations arerestored.

3. In a wireless receiver at a radio receiving station having signalsand disturbances arriving thereat, means for controlling the signallevel, said means comprising a compressor having such a time constant asto reduce, at the receiving station, the amplitude fluctuations in thesignal caused by modulation, an amplitude filter for reducing thedisturbing amplitudes to approximately the highest signal amplitude, acontrol element for enlarging the signal contrast, whereby the amplitudefluctuations of the signal, which were evened out by the said signallevel control, are compensated for, so that the original amplitudefluctuations are restored, said amplitude filter at the receivingstation comprising an electron discharge amplifier tube having a gridbias which is so small that signals can swing the grid thereof oversubstantially the whole straight part of the tube characteristic, meansfor rectifying the transmission from said first control means, which isin the form of a high frequency wave and for applying the rectified highfrequency wave to the grid of said amplitude filter, the positiveterminal of said rectifying means being connected to the commonpotential point of the receiver so that the rectified high frequencywave swings out over the negative range of potential and is impressedupon the grid of the amplitude filter, whereby individual voltageimpulses of larger amplitude swing so far into the negative range of thechar- 7 acteristic of said amplitude filter that they are substantiallynot amplified.

4. The device according to claim 2, wherein 'said'amplitude filterconsists of an electronic amplifier tube having at least two grids, oneof said grids being connected as control grid and the other of saidgrids being connected as screening grid, the voltage applied to saidanode not exceeding one; half of the voltage applied to said

